1. Field of the Invention
The present invention relates to analog voltage driver circuits configured for supplying analog voltages within an integrated circuit to an output pad for testing using an external testing device such as a high-speed oscilloscope.
2. Background Art
High-speed input/output (I/O) devices, such as physical layer transceivers (PHYs) or I/O devices, are configured for outputting digital signals on a bus in the form of analog signals having prescribed voltages according to prescribed voltage specifications. Highly integrated mixed signal designs (e.g., integrated bridge devices) for such I/O devices typically have multiple voltage domains and clock domains for both digital and analog circuitry in order to interface between different bus systems, for example a HyperTransport™ link and a PCI-X bus. For example, such I/O devices have relatively complex analog circuitry such as phase locked loops (PLL), etc. Hence, analog operations within the I/O devices need to be precisely controlled.
Process variations that are encountered during manufacture of such high-speed I/O devices into silicon-based integrated circuits often result in integrated circuits that exhibit voltage and/or timing variations due to such process variations, as well as variations caused by temperature changes or variations in supply voltages. Hence, manufacture of such I/O devices as integrated circuits requires testing and evaluation of the integrated circuit to ensure precise control of the analog signals according to the prescribed voltage specifications.
In such highly integrated mixed signal designs, however, it is quite difficult to access internal high-impedance, low capacitance analog nodes for test or debug in a flip-chip assembled die (e.g. a die having conductive bumps on a bond pad). In particular, the physical structure of an assembled die (e.g., a flip-chip assembled die) severely restricts the availability of output pads that can be used for probing internal analog voltages. Moreover, attempts to read an analog voltage signal by digitizing the analog voltage signal into a digital value, and outputting the digital value via a digital output port are not practical due to power consumption, cost of silicon area, and the bandwidth of the analog signal.
Hence, access to a large number of different analog nodes within the integrated circuit is impractical.